(1) Field of the Invention:
The present invention relates to a synthetic crystal for an artificial gem. More particularly, the invention relates to a synthetic single crystal which simulates natural alexandrite in the hue and color change characteristics.
(2) Description of the Prior Art:
Research has heretofore been expended to artificially grow alexandrite having a crystal structure of chrysoberyl (BeO.Al.sub.2 O.sub.3). A problem to be solved in commercially manufacturing an artificial gem of alexandrite is how to manufacture the gem such that the hue and color characteristics resembling those of natural alexandrite produced in the Ural Mountain Region, which is now regarded as having highest quality and value. More specifically, it is necessary to obtain a product which shows a green or bluish green color under natural light but shows a red or reddish violet color under the light of an electric lamp (tungsten light).
As means for solving this problem, the specification of U.S. Pat. No. 3,912,521 proposes a method in which chromium (Cr) and iron (Fe) are incorporated in aluminum oxide (Al.sub.2 O.sub.3) and beryllium oxide (BeO) as the main components. Chromium is an additive element imparting color change characteristics to the resulting synthetic single crystal. However, a single crystal obtained by addition of chromium alone is much inferior to natural alexandrite produced in the Ural Mountain Region in the hue and color change characteristics, and the commercial value of this crystal as an artificial gem is very low. In the above-mentioned U.S. patent specification, therefore, iron is used as a second additive element in an attempt to obtain a desired hue by mingling the color of chromium with the color of iron.
This known single crystal for an artificial gem as taught in the above-mentioned patent is improved to some extent in the color change characteristics over a product formed by using chromium alone as an additive, but its color under natural light is slightly yellowish or brownish green, which is due to the presence of iron, and is therefore inferior to a green color possessed by natural alexandrite produced in the Ural Mountain Region. Furthermore, when this known single crystal is exposed to tungsten light and the color change is examined, it is seen that the color is changed to reddish violet, but the sensitivity to the color change or the degree of the color change is degraded because of influences of the above-mentioned yellowish or brownish hue. Accordingly, it cannot be said that this crystal is comparable to natural alexandrite produced in the Ural Mountain Region with respect to the color change characteristics. In short, a satisfactory synthetic single crystal for an alexandrite gem has heretofore not been developed.